This attribute shows up in the output XML infoset from parsing, presumably one must specify certain options to the CLI/API in order for this attribute to be present.

That said - this means this attribute is really publicly visible, and not just some internal daffodil thing. So it should be the in the daffodil external namespace (which uses prefix "daf" by convention), not the internal namespace.

Note: Wiki page about "Daffodil and the DFDL Infoset" needs to be updated when this change is made.